Referring to FIG. 1 of the drawings, a timing diagram for a Manchester-II encoded Auxiliary Channel synchronization pattern and stop condition for a DISPLAYPORT Standard v1.1a (Feb. 23, 2008) from the Video Electronics Standards Association (VESA), which is incorporated by reference herein for all purposes. In this Manchester-II encoding scheme, a transition from low to high corresponds to a “1,” while a transition from high to low corresponds to a “0.” As can be seen, the synchronization pattern is 16 consecutive zeros followed by a two bit high period (2 μs for a bit rate of 1 Mbps) and 2 bit low period (2 μs for a bit rate of 1 Mbps). Additionally, this synchronization pattern is preceded by 0 to 16 consecutive pre-charge “0's.” This is done so that the common mode of the ac coupled clockless data lines settle before the actual transaction begins. Data transmission follows synchronization low period and is discontinued with the stop condition (a two bit high period followed by a two bit low period).
Since the Manchester-II encoded serial input is asynchronous, a decoder should be able to decode the data, independent of clock jitter and clock frequency variance. To accomplish this, however, a narrow unit interval (UI) is generally required by the DISPLAYPORT standard. In particular, a variance in UI width within a particular transaction is generally limited to ±5% of the width of the transaction; typically, for a nominal UI width of 0.5 μs, the window is generally between 0.4 μs to 0.6 μs. Due to the source clock variance used to generate the signal transaction relative to the asynchronous and highly process/voltage/temperature variable sampling clock, it can be difficult to distinguish between a synchronization pattern and a UI as well as between UI and 2UI. Additionally, one of the most difficult problems is to distinguish between a “sync-start-low” and 4UI's or 5UI's due to the lack of transition during the continuous low, which will generally determine which is the first bit of data. Another difficulty is the proper receipt of the Auxiliary transaction when determining the transmitted UI such that the transaction can be received properly is the variability in the number of pre-charge zeros seen by the receiver as well as the undesirable variability in the duration of the pre-charge zeros. Ideally, the pre-charge zeros are not used for UI calculation. It should be noted that improperly distinguishing any transmitted information will result in improper system operation and is highly undesirable. Therefore, there is a need for a highly reliable Manchester-II decoder implementation that is compatible with the DISPLAYPORT standard.
Some examples of conventional devices are U.S. Patent Pre-Grant Publ. No. 2009/0027401 and European Patent No. 1473695.